This invention relates to an improved method and apparatus for monitoring the flow of particulate solids through a screw conveyor which employs a source of radiant energy and a radiation detector disposed so that radiation is passed into a bed of flowing solids in a conveyor and the portion of the radiation that is not absorbed or scattered by the solids or the apparatus is picked up by the detector, thereby energizing an electrical circuit connecting the detector and a recorder. Such apparatus is known as a nuclear weightometer.
U.S. Pat. No. 2,953,682 issued Sept. 20, 1960 to G. E. Frank et al. discloses use of radiant energy to detect changes in viscosity and mass of a liquid as it is stirred by rotation of a horizontal screw in a cylindrical reaction vessel. Radiation sources direct beams through both raised and depressed edges of liquid as it is being stirred by the screw. A change in the difference of the outputs of collectors in each radiation path indicates a change in viscosity. A change in the sum of the outputs indicates a change in mass. Elimination of the effect of random and periodic disturbances is provided by integration of readings obtained from the detector signals (Column 4, line 75 to Column 5, line 17).
In the process of U.S. Pat. No. 3,036,214 issued May 22, 1962 to R. C. Forney et al. radiant energy is used to indicate the progress of polymerization of liquid materials as it flows through a horizontal cylindrical chamber. The material is mixed by interrupted screw flights and intervening transverse screens. Viscosity changes, as indicated by relative vertical displacement of portions of the surface of the liquid at opposite sides of the vessel axis, are determined by beaming radiant energy, between two screw flights (Column 3, lines 54 to 66), to pass through part of the displaced liquid.
U.S. Pat. No. 3,518,425 issued June 30, 1970 to C. L. Gruenwald discloses nuclear radiation means for measuring the flow of solid material through a screw conveyor. The radiation source and detector of this patent are spaced to span the screw conveyor at right angles to the direction of flow of the solids. Both source and detector, which are parallel to one another, are substantially as long as the diameter of the conveyor. It is noted by the inventor that the orientation of the detector source relative to the free surface of the material in the screw is most significant. He demonstrates and claims orientation in which the signal transmitted from the detector to the recording means is linearized electronically more satisfactorily than it is in other positions, thereby recording more accurately the weight of material moving through the conveyor. The preferred orientation is obtained when the elongated source and elongated detector are supported in a plane transverse to the longitudinal extent of the screw conveyor, parallel to each other, and in a plane substantially perpendicular to the free surface of the material that is carried by the screw conveyor. As implied by the inventor's reference to U.S. Pat. No. 3,278,747 issued Oct. 11, 1966 to P. E. Ohmart and as shown in FIG. 1 therein, the radiation path spans only a small part of one pitch length of the conveyor screw.
In each of the foregoing nuclear detection means, only material in a narrow segment of one pitch length of the rotating screw is scanned. In the case of U.S. Pat. No. 3,518,425, the transverse radiation beam scans the full width of the screw conveyor with the result that the detector signal represents all the moving material that moves through the screw conveyor. Advantages of this arrangement are offset by the manner in which particulate material is moved in a screw conveyor. As a bed of this material is moved in a horizontal direction, it becomes deeper on the ascending side of the screw and it also climbs on the pushing surface of the screw blade, thus providing a depth differential along each screw pitch length. We have discovered that, by scanning the deeper portion of the material on the lift side of the screw for at least about a full pitch of the screw flight, the detector continuously receives a composite radiation signal from a representative portion of the material. A narrow transversely placed detector, as shown in the prior art, senses these variations in depth and provides a fluctuating signal that requires subsequent electronic averaging to eliminate irregular recording of uniform overall flow. These fluctuating signals are eliminated by the process and apparatus of the present invention.